Thermometers

A very precise way to measure the temperature of an object would be by summing the kinetic energy of its constituent particles. Unfortunately, this method is not feasible in the real world as even tiny objects contain billions of particles. Therefore, to obtain a quantitative measurement of temperature we always measure temperature through some other proxy.

This can, for example, be the expansion of the length of a column of mercury in a glass capillary, the bending of a bimetallic strip or a change in the electrical resistivity of a material or some other physical change. An instrument that measures the temperature of a material by measuring some other physical change is called a thermometer and since the measurement is dependent on some physical change, each type of thermometer is limited to functioning over a limited temperature range.